The cellular industry is constantly seeking new ways for improving how people communicate. With the introduction of EDGE (Enhanced Data rates for GSM Evolution) technology [1] for GSM (Global System for Mobile communications) and the HSDPA (High Speed Downlink Packet Access) [2] for WCDMA (Wideband Code Division Multiple Access) the data communication in cellular terminals has reached new heights. The IMS (IP Multimedia Subsystem) [3] gives new opportunities to create additional interesting IP based services, such as PoC (Push to talk Over Cellular) [4]. MBMS (Multimedia Broadcast Multicast Service) [5] is yet another feature which allows for efficient deployment of broadcast and multicast services, such as mobile TV. This is true as MBMS offers true broadcast and multicast even over the radio. For example users, or actually user terminals, in GSM/(E)GPRS ((Enhanced) General Packet Radio Services) would listen to the same time slots carrying the MBMS data channel.
With the above in mind it is obvious that cellular system used for traditional mobile telecommunication and data communication is starting to become a highly attractive alternative to the analogue PMR (Public Mobile Radio) and PSR (Public Safety Radio) system, used by e.g. police departments, fire departments, ambulance personnel, security and transport companies.
One service included in a PMR or PSR system based on commercial available cellular technologies, as given above, would be PoC using MBMS to support large communication groups. This would enable an attractive PMR/PSR service highly useful for example by firemen, taxi drivers and the other above-identified potential users.
When a user has made a multicast or broadcast call to multiple users of the IMS service/application, such as PoC service, some of these listening users might want to respond to the message by sending a multicast or broadcast message himself/herself. In MBMS, uplink resources useful for these users must first be requested before the data can be transmitted to the base station and multicast/broadcast to the other users of the IMS service. In the art, at least one RACH (Random Access Channel) channel is available in the cell and can be used by user terminals for transmitting uplink access request message. Access to RACH channel is commonly done according to standard procedure as described in [6].
The document [6] specifies that user terminal shall send UPLINK ACCESS messages on the voice group call channel with the appropriate establishment cause. The first UPLINK ACCESS message shall be transmitted by the user terminal with a random delay between 0 and 20 ms. If the user terminal is not granted any uplink resources (uplink access), the UPLINK ACCESS messages shall be repeated after a further period of 100 ms plus a random delay between 0 and 20 ms. The UPLINK ACCESS messages contain a random reference which is drawn randomly from a uniform probability distribution. The UPLINK ACCESS messages repetitions shall contain the same random reference as the one contained in the first message.
Although there is randomness in generation of the access request, the collision probability i.e. blocking on the RACH, is considerable when employing multicast/broadcast signalling for IMS services, such as PoC. This problem will be particularly prominent at the end of a previously sent downlink (multicast/broadcast) message. At that point of the communication session, a large amount of the participating users might want to respond to the downlink message and all these users would then almost simultaneously or at least within a small time window transmit uplink access requests on the RACH channel(s).
One scenario example is when a commander in chief gives instructions which require people to respond. Another one is the warnings of hazards that can be shouted out during firemen putting out fires, etc. A third example is during major sport events such as world football (soccer) cup, when both audience and police can use PoC/PMR/PSR for communication.
There will therefore be RACH access limitations due to the large number of groups members located in the same area (cell). Since many channel access requests are transmitted during a short time period, for example after MBMS data transmission, there may be a problem to get anything through and no or few of the group members will be allocated any uplink resources. As a result only few, if any, of the members wanting to respond to a message or transmit data will be able to do this.